Rail Fence Cipher

The rail fence cipher (also called a zigzag cipher) is a transposition cipher in which the message is split across a set of rails on a fence for encoding. The fence is populated with the message's characters, starting at the top left and adding a character on each position, traversing them diagonally to the bottom. Upon reaching the last rail, the direction should then turn diagonal and upwards up to the very first rail in a zig-zag motion. Rinse and repeat until the message is fully disposed across the fence. The encoded message is the result of concatenating the text in each rail, from top to bottom.

From wikipedia, this is what the message WE ARE DISCOVERED. FLEE AT ONCE looks like on a 3-rail fence:

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . A . . . I . . . V . . . D . . . E . . . N . .
-------------------------------------------------
             WECRLTEERDSOEEFEAOCAIVDEN

The message can then be decoded by re-creating the encoded fence, with the same traversal pattern, except characters should only be added on one rail at a time. To illustrate that, a dash can be added on the rails that are not supposed to be populated yet. This is what the fence would look like after populating the first rail, the dashes represent positions that were visited but not populated.

W . . . E . . . C . . . R . . . L . . . T . . . E
. - . - . - . - . - . - . - . - . - . - . - . - .
. . - . . . - . . . - . . . - . . . - . . . - . .

It's time to start populating the next rail once the number of visited fence positions is equal to the number of characters in the message.

Code

railFenceCipher.js

/**
 * @typedef {string[]} Rail
 * @typedef {Rail[]} Fence
 * @typedef {number} Direction
 */

/**
 * @constant DIRECTIONS
 * @type {object}
 * @property {Direction} UP
 * @property {Direction} DOWN
 */
const DIRECTIONS = { UP: -1, DOWN: 1 };

/**
 * Builds a fence with a specific number of rows.
 *
 * @param {number} rowsNum
 * @returns {Fence}
 */
const buildFence = (rowsNum) => Array(rowsNum)
  .fill(null)
  .map(() => []);

/**
 * Get next direction to move (based on the current one) while traversing the fence.
 *
 * @param {object} params
 * @param {number} params.railCount - Number of rows in the fence
 * @param {number} params.currentRail - Current row that we're visiting
 * @param {Direction} params.direction - Current direction
 * @returns {Direction} - The next direction to take
 */
const getNextDirection = ({ railCount, currentRail, direction }) => {
  switch (currentRail) {
    case 0:
      // Go down if we're on top of the fence.
      return DIRECTIONS.DOWN;
    case railCount - 1:
      // Go up if we're at the bottom of the fence.
      return DIRECTIONS.UP;
    default:
      // Continue with the same direction if we're in the middle of the fence.
      return direction;
  }
};

/**
 * @param {number} targetRailIndex
 * @param {string} letter
 * @returns {Function}
 */
const addCharToRail = (targetRailIndex, letter) => {
  /**
   * Given a rail, adds a char to it if it matches a targetIndex.
   *
   * @param {Rail} rail
   * @param {number} currentRail
   * @returns {Rail}
   */
  function onEachRail(rail, currentRail) {
    return currentRail === targetRailIndex
      ? [...rail, letter]
      : rail;
  }
  return onEachRail;
};

/**
 * Hangs the characters on the fence.
 *
 * @param {object} params
 * @param {Fence} params.fence
 * @param {number} params.currentRail
 * @param {Direction} params.direction
 * @param {string[]} params.chars
 * @returns {Fence}
 */
const fillEncodeFence = ({
  fence,
  currentRail,
  direction,
  chars,
}) => {
  if (chars.length === 0) {
    // All chars have been placed on a fence.
    return fence;
  }

  const railCount = fence.length;

  // Getting the next character to place on a fence.
  const [letter, ...nextChars] = chars;
  const nextDirection = getNextDirection({
    railCount,
    currentRail,
    direction,
  });

  return fillEncodeFence({
    fence: fence.map(addCharToRail(currentRail, letter)),
    currentRail: currentRail + nextDirection,
    direction: nextDirection,
    chars: nextChars,
  });
};

/**
 * @param {object} params
 * @param {number} params.strLen
 * @param {string[]} params.chars
 * @param {Fence} params.fence
 * @param {number} params.targetRail
 * @param {Direction} params.direction
 * @param {number[]} params.coords
 * @returns {Fence}
 */
const fillDecodeFence = (params) => {
  const {
    strLen, chars, fence, targetRail, direction, coords,
  } = params;

  const railCount = fence.length;

  if (chars.length === 0) {
    return fence;
  }

  const [currentRail, currentColumn] = coords;
  const shouldGoNextRail = currentColumn === strLen - 1;
  const nextDirection = shouldGoNextRail
    ? DIRECTIONS.DOWN
    : getNextDirection(
      { railCount, currentRail, direction },
    );
  const nextRail = shouldGoNextRail ? targetRail + 1 : targetRail;
  const nextCoords = [
    shouldGoNextRail ? 0 : currentRail + nextDirection,
    shouldGoNextRail ? 0 : currentColumn + 1,
  ];

  const shouldAddChar = currentRail === targetRail;
  const [currentChar, ...remainderChars] = chars;
  const nextString = shouldAddChar ? remainderChars : chars;
  const nextFence = shouldAddChar ? fence.map(addCharToRail(currentRail, currentChar)) : fence;

  return fillDecodeFence({
    strLen,
    chars: nextString,
    fence: nextFence,
    targetRail: nextRail,
    direction: nextDirection,
    coords: nextCoords,
  });
};

/**
 * @param {object} params
 * @param {number} params.strLen
 * @param {Fence} params.fence
 * @param {number} params.currentRail
 * @param {Direction} params.direction
 * @param {number[]} params.code
 * @returns {string}
 */
const decodeFence = (params) => {
  const {
    strLen,
    fence,
    currentRail,
    direction,
    code,
  } = params;

  if (code.length === strLen) {
    return code.join('');
  }

  const railCount = fence.length;

  const [currentChar, ...nextRail] = fence[currentRail];
  const nextDirection = getNextDirection(
    { railCount, currentRail, direction },
  );

  return decodeFence({
    railCount,
    strLen,
    currentRail: currentRail + nextDirection,
    direction: nextDirection,
    code: [...code, currentChar],
    fence: fence.map((rail, idx) => (idx === currentRail ? nextRail : rail)),
  });
};

/**
 * Encodes the message using Rail Fence Cipher.
 *
 * @param {string} string - The string to be encoded
 * @param {number} railCount - The number of rails in a fence
 * @returns {string} - Encoded string
 */
export const encodeRailFenceCipher = (string, railCount) => {
  const fence = buildFence(railCount);

  const filledFence = fillEncodeFence({
    fence,
    currentRail: 0,
    direction: DIRECTIONS.DOWN,
    chars: string.split(''),
  });

  return filledFence.flat().join('');
};

/**
 * Decodes the message using Rail Fence Cipher.
 *
 * @param {string} string - Encoded string
 * @param {number} railCount - The number of rows in a fence
 * @returns {string} - Decoded string.
 */
export const decodeRailFenceCipher = (string, railCount) => {
  const strLen = string.length;
  const emptyFence = buildFence(railCount);
  const filledFence = fillDecodeFence({
    strLen,
    chars: string.split(''),
    fence: emptyFence,
    targetRail: 0,
    direction: DIRECTIONS.DOWN,
    coords: [0, 0],
  });

  return decodeFence({
    strLen,
    fence: filledFence,
    currentRail: 0,
    direction: DIRECTIONS.DOWN,
    code: [],
  });
};

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